Stress-corrosion cracking (SCC) has been found to be the cause of failure of 48-in. (1219.2-mm) and 30-in. (762-mm) diameter manganese bronze gate valve bodies, a 48-in.-diameter shaft cap, and 1½-in. (38-mm)-diameter nuts. All of these in-service failures occurred in New York City distribution shafts for Water Tunnels Nos. 1 and 2. The valves and shaft cap, which were made of Copper Development Association (CDA) Alloy C86500, failed in predominantly transgranular mode with some intergranular cracking. The nuts, made of CDA Alloy C86300, cracked in an intergranular mode only.

With numerous manganese bronze castings presently in service in all 37 water distribution shafts in New York City, a stress-corrosion cracking test program has recently been initiated. The specimens used were of the modified compact wedge-loaded type made of CDA Alloy C86500. The purpose of the test was not only to find which distribution shafts contained an aggressive SCC environment, but to identify any corrosive agents in the shafts. All 37 shaft environments produced cracking in one or more of the specimens tested in 15 to 63 days. Most of the cracking occurred with the pH in the range from 7.5 to 9.5.

The spillage of elemental mercury from flowmeters in the shafts plus the introduction of chemicals used for mercury cleanup accelerated the SCC investigation. Mn-bronze specimens were tested in jars containing ammonia, mercury, water, trisodium phosphate, and commercial mercury cleaning solutions. All chemical induced cracking in the specimens in a range from 1 to over 300 days, except mercury and trisodium phosphate.

Scanning electron microscopy was used to characterize the crack pattern of several in-service failures and SCC test specimens. Energy-dispersive analysis was used to identify the corrosion products on the crack surfaces.